Reference is made to my two above identified patents for further background of this invention, and to the art of record set forth therein.
Applicant has discovered that it is possible to connect a special shaft driven grinder device to a shaft, support the grinder device within a pipe while axially aligning the center of the grinder device respective to the longitudinal central axis of the pipe, and to reciprocate the grinder device along the length of the pipe while the grinder device is rotated by the shaft and thereby remove a plastic lining along with a small amount of the interior metal part of the pipe surface with precision and thereby provide the pipe with a uniform inside diameter that is cleaned of debris and therefore is ideal for receiving a coating of plastic material. In this disclosure, the terms pipe, casing, and pipe section are considered equivalent.
In the art of plastic coating the interior surface of joints or sections of pipe, it has been customary to re-use old plastic coated pipe joints by thoroughly cleaning the old pipe joints to remove the old plastic coating. This is accomplished by first firing them in a large oven to burn the old coating from the pipe interior. Then the pipe is cooled and the dirty pipe interior is sand blasted to remove all foreign substances and to expose uncontaminated metal along the entire length of the pipe to which the new coating will satisfactorily adhere. Next the pipe is coated using a known coating process. This usually requires heating the pipe above the melting point of the plastic and then the hot pipe interior is contacted with plastic powder which melts into a uniform coating. Several successful processes for powder coating are set forth in Gibson U.S. Pat. Nos. 4,089,998 and 4,420,508; to which reference is made for further background. There are many other Patents related to the plastic coating of pipe, such as those cited in the Gibson Patents.
Irregularities often randomly occur on the inside wall surface of pipe sections which preclude the pipe from being used in deep wells or for in a cross-country pipe line. This unacceptable pipe is referred to as "un-drifted pipe", or pipe that has an irregular interior diameter and therefore is too small to accept the drift throughout the length thereof. This un-drifted pipe usually is perfectly good except for these small excursions from ideal diameter. Because of the irregularities, the pipe cannot be used downhole since an oil company must be assured that the entire casing or string of pipe is of a minimum diameter to avoid the danger of subsequently sticking a tool string, or other device, downhole in the pipe or in the casing. A stuck object, called a fish, is usually considered a catastrophe in the oil field and requires an expensive fishing job in order to retrieve the fish from the borehole.
Moreover, many new pipe sections are rejected because of small, sharp, metallic imperfections which can protrude through a plastic coating to cause newly coated pipe to be rejected by quality control. The pipe section must then be processed all over again, just because of such small imperfections. Too many rejects can bankrupt a company, especially a company that must depend upon the old fashioned method of cleaning the used pipe. Hence, it is sometime advantageous to clean the inside surface of new pipe, as well as the used pipe, so as to avoid the probability of having to process the pipe a second time.
The E.P.A. (Everyone in the oil business is familiar with this term) dislikes contamination of our atmosphere with noxious gases, and especially those emitted from an oven during the processing of used pipe, wherein the plastic coating is combusted or burned from the interior of the pipe. This is considered wasteful, expensive, and a health hazard. Applicant has discovered that used plastic coated pipe can be economically reclaimed without the necessity of burning the coating in an oven. This is accomplished by running a special grinding tool, made in accordance with this invention, through the pipe interior to remove the old coating and to additionally prepare the interior metal surface of the pipe to receive the new plastic coating. The grinding tool, together with a considerable amount of support equipment, provides a method and apparatus for drifting and cleaning new or used pipe so that the expensive un-drifted pipe can be reclaimed and properly employed as drifted pipe that is free of imperfections. The savings in labor and equipment afforded by this invention are considerable, a complete heat cycle is avoided, there are no noxious gases released into the atmosphere, the sandblasting of the pipe interior can be omitted, and the removed plastic and metal can be recycled.